Descent of the testes from a position near the kidneys into the lower abdomen and/or scrotum during development is a fundamental process for most mammals that has been linked to improved testicular function. The INSL3 ligand and its cognate receptor (LGR8/RXFP2) are key to the gubernaculum-driven relocation of the testes from the kidneys to the lower abdomen with functional forms of their encoded genes located in conserved regions of the genomes of a range of eutherian mammals. However, the role of INSL3 in testicular descent in marsupials is not clear, and is important given the different mechanism of scrotum determination (X-linked determination) in marsupials compared to eutherians. We have compared functional forms of INSL3 and LGR8 from a broad range of marsupial orders, including Microbiotheridae, Dasyuromorphia, Notoryctemorphia and Diprotodontia, and show testicular expression and localisation of these genes during development in an Australian marsupial, the dunnart (Sminthopsis crassicaudata). Taken together, these data show that INSL3/LGR8-mediated testicular descent is a common feature of marsupials, all of which have either partial or completely descended testes. Thus, the genetic mechanism mediating this fundamental trait evolved in therian mammals and suggests that testicular descent, at least to the abdominal wall, was an ancestral feature of the group.